1. Field of the Invention
This invention relates to anti-gel and anti-block components useful for reducing optical degradation phenomena and discoloration in extruded film.
2. Description of the Prior Art
Olefin polymers such as polyethylene have been used for a number of years for the preparation of films by extrusion processes. Unlike high pressure, free radical polymerized polyethylenes, low pressure, low density polyethylenes prepared in the presence of transition metal catalysts such as the Ziegler-Natta catalysts frequently develop gel-like defects when extruded, e.g., when extruded as blown films. In a blown film process, polyethylene pellets or granules are melted and extruded from an annular die to form a film envelope. In the extruder and die system, there are typically "dead spots" or "hangup areas" where molten polyethylene does not move rapidly, resulting in long heat histories for portions of the melt. As portions of the polymer are flushed from these hangup areas, they cause optical degradation phenomena in the film, known as "pinstriping" and "gel streaking".
By "pinstriping" is meant haze bands resulting from accumulations of micro-gels which are aligned in the machine direction during the film forming process. By "gel-streaking" is meant surface roughness in the form of the inclusion, either singularly or in rows, of "v-shaped" gels ("arrowheads" or "chevrons"), streaks of soft gels, or, in extreme cases, film texture completely permeated by soft gels. Such optical degradation phenomena not only detract from film optical and mechanical strength properties, but also can cause severe problems with respect to maintaining bubble geometry during the film forming process.
Because of the fact that the extrusion processes usually involve the use of relatively high extrusion temperatures and prolonged extrusion times, in commercial practice, various anti-oxidants, such as hindered phenolics, are admixed with the olefin polymer, to prevent undesirable degradation of the polymer during the preparation of the film. When such anti-oxidants are used, however, with the film forming grades of olefin polymers in certain types of extrusion equipment, the anti-oxidants appear to pronounce, if not create, gel-streaking and pinstriping in the products thus formed.
Blocking of olefin polymer films is a well known phenomenon. By blocking is meant the tendency of films or sheets to stick or adhere to each other or to other surfaces whenever adhesion is undesirable. Various anti-block agents are commonly incorporated into film forming polymers or applied to film surfaces to reduce blocking. For example, stearamide, stearoguanamine, metal salts of fatty acids such as calcium stearate; sodium dioctylsufosuccinate; finely divided silica or calcium carbonate; alkylamines and alkyl quaternary ammonium compounds are anti-block agents which have been incorporated into the film forming polymer before extrusion.
Many solutions have been proposed to eliminate gel-streaking and pinstriping. For example, it is known to physically remove the chloride catalyst residue, prior to extrusion, by leaching the reaction product of polymer and chloride catalyst residue with alcohols, aqueous acid, water, or treating the polymer and catalyst with propylene oxide followed by an alcohol or water wash and the like. Such treatments usually produce white polymers initially, but a yellow or tan color returns when the polymers are subjected to molding and/or heating operations. In order to avoid this undesirable color formation, it is usually necessary to use a second or even a third clean-up procedure requiring the use of large quantities of deactivating materials. Another practice in the art involves drying the after-treated polymer prior to fabrication. Because they usually must be repeated several times in order to obtain a polymer having acceptable color upon exposure to heat, such cleanup procedures are both expensive and time consuming. Illustrative of such prior art treatments are those disclosed in U.S. Pat. Nos. 3,925,341; 3,962,199; 3,247,351; 4,029,877; 4,117,219; 3,299,027; 3,923,760; 3,308,105; and 4,098,990.
Other treatments disclosed in the prior art involve the addition of compounds to the polymer prior to fabrication in order to complex with the harmful components in the chloride catalyst residue and deactivate them. Illustrative of these prior art treatments are those disclosed in Canadian Pat. No. 961,998, U.S. Pat. No. 4,013,622 and in U.S. Pat. No. 3,773,743.
U.S. Pat. No. 3,773,743 discloses a method for improving the stability and color of olefin polymers by deactivating their Ziegler-Natta chloride catalyst residues. This method involves high temperature (190.degree. C.-250.degree. C.) processing with hydroxyl compounds (H.sub.2 O and primary alcohols) and with an organic base such as an alkyl amine; aryl amine; Li, Ca and Zn salts of carboxylic acids; trialkyl phosphites; and metal alkoxides. The concentration disclosed for the hydroxy source is the range of 0.5 to 1.5 weight percent and a concentration of 50 to 2500 parts per million (ppm) is disclosed for the organic base. At the processing temperatures disclosed, some of the organic bases and the hydroxy sources would be volatile and cause foaming of the product if the processing were not done in a way to remove volatiles, e.g., as in devolatilizing extruders.
Canadian Pat. No. 961,998 and U.S. Pat. No. 4,013,622 disclose the addition of polyalkylene glycol having a molecular weight between 100 and 20,000 to film grade olefin polymers to prevent gel streaking during film extrusion processes. In the practice as taught in the Examples--Superfloss with a pH of about 9 to 11 is used as the anti block--the extruded film made according to Canadian Pat. No. 961,998 discolors in a few weeks even at ambient temperatures, i.e., it turns a yellowish color which is, of course, undesirable.
The potential cause for discoloration on ageing in extruded films or molded articles is generally recognized as related to the presence of hindered phenolic anti-oxidants. Discoloration generally is believed to result from the reaction or breakdown products of such anti-oxidants. Exposure of these anti-oxidant stabilized compositions to air pollution in the form of oxides of nitrogen can promote discoloration. Thermal abuse in processing, especially in the presence of prodegradants, can lead to the build-up of chromophoric quinoid structures from break-down of the hindered phenolic anti-oxidants.
Prodegradants can be additives used to control surface slip or release properties, anti-static properties, and gel streaking tendencies. For example, prodegradants act to provide hydroperoxides to accelerate auto-oxidation. Polyethylene glycols used to control gel streaking and unsaturated fatty amides used for slip control can act with thermal abuse as prodegradants. Trace metals from inorganic anti-blocks or fillers can act as reduction-oxidation catalysts to accelerate hydroperoxide decomposition, especially at low temperatures where the peroxide decomposition is the rate limiting step. At such low temperatures, normally volatile, anti-oxidant reaction or break-down products can accumulate, promoting discoloration. Moreover, inorganic additives or fillers such as finely divided silica with iron contamination can interact with the hindered phenolic anti-oxidant to form chromophoric, phenolate species.